Packaging materials comprising biaxially stretched polyester films excellent in heat resistance, mechanical strength, impact resistance and oil resistance are used for packaging food to be subjected to a heat sterilization treatment such as a boiling treatment or a retorting treatment. Examples of the packaging configurations of such packaging materials include: PET/CPP, PET/ON/CPP, PET/Al/CPP, PET/Al/ON/CPP and PET/ON/Al/CPP, wherein PET, Al, CPP and ON stand for polyethylene terephthalate, aluminum foil, unstretched polypropylene film and biaxially stretched nylon film, respectively.
Such packaging materials are required to be linearly tearable (hereinafter, being linearly tearable is referred to as “linear tearability” as the case may be) when the packages are opened. However, in the case where packaging bags are formed of heretofore known packaging materials using common biaxially stretched polyester films, when such packaging bags are opened, the packaging bags are not linearly torn and consequently the contents of the packaging bags are scattered or other troubles occur as the case may be.
As a technique to solve such problems, JP2002-080705A discloses a highly adhesive film having a highly adhesive layer disposed on a biaxially stretched film in which a modified polybutylene terephthalate (modified PBT) containing polybutylene terephthalate (PBT) and polytetramethylene glycol (PTMG) is mixed with PET. When a packaging material is prepared from such a highly adhesive film, the laminate strength and the linear tearability of the packaging material can be improved.
Additionally, in a heretofore known technique, a polyester resin is used in the highly adhesive layer of a polyester film. For the purpose of improving the heat resistance and the adhesiveness of the highly adhesive layer, a crosslinking agent such as melamine is added to the polyester resin constituting the highly adhesive layer (for example, JP08-311221A, paragraph 0057).
However, it has been revealed that in a laminated film including as a substrate a film having linear tearability, when the laminate strength after a boiling treatment or a retorting treatment is degraded and consequently the delamination tends to occur, unfortunately the linear tearability intrinsically belonging to the substrate film is not sufficiently displayed and the linear tearability is degraded in the laminated film.
On the other hand, formaldehyde is generated from the melamine resin used as the crosslinking agent, and formaldehyde is known to cause sick house syndrome or sick school syndrome. In the packaging industries, there is a trend to reduce the addition of melamine resin for the purpose of improving the work environment at the time of manufacturing products and other purposes. However, it has been revealed that the laminate strength is insufficient without melamine resin, when boiling treatment or retorting treatment is conducted under the more severe conditions recently required for the packaging materials.